Modern Storage Structures: Refrigerated and Controlled Atmosphere Storage

Fresh fruits and vegetables don’t stop living after harvest. They continue to breathe, ripen, and eventually spoil – a process that causes massive postharvest losses worldwide. To combat this, modern storage structures such as refrigerated storage and controlled atmosphere (CA) storage have become essential tools in the food processing industry. These technologies work by manipulating the storage environment – temperature, humidity, and even the air itself – to slow biological activity and keep produce fresh for weeks or even months longer than traditional methods allow.

Why modern storage matters 🔗

After harvesting, fruits and vegetables undergo respiration – they consume oxygen and release carbon dioxide, much like we do when we breathe. This biological process breaks down stored sugars and other compounds, gradually reducing quality and leading to spoilage. Alongside respiration, transpiration (loss of moisture) and ethylene production (a natural ripening hormone) further accelerate deterioration.

Traditional open-air or ambient storage cannot address these processes effectively, resulting in significant losses – especially in tropical and developing regions. According to research published in Discover Food, postharvest losses of fresh produce are a major issue in countries with inadequate cold storage infrastructure, forcing farmers to sell below market prices. Modern storage structures directly tackle these challenges by creating carefully controlled environments.

Refrigerated storage: the backbone of postharvest preservation 🔗

Refrigerated storage – also called cold storage – is the most widely used modern method for preserving perishable produce. At its core, it works on a straightforward principle: lowering temperature slows biological activity. When fruits and vegetables are kept at low temperatures, their respiration rate drops, ethylene production decreases, and microbial growth is inhibited.

How refrigerated storage works 🔗

A refrigerated storage facility uses a mechanical refrigeration system to remove heat from the storage environment and maintain a consistent low temperature. According to PostHarvest Technologies, cold storage facilities go beyond just providing cool rooms – they now include supply chain analytics, controlled atmospheres, and data-driven monitoring.

There are two main refrigeration principles used in cold storage units:

Vapour Absorption System (VAS): This system has a higher initial cost but is more economical to operate over time. It works well for products that need storage above 10°C, such as mangoes and seeds. However, many fruits and vegetables require temperatures below 10°C, which limits its applicability.

Vapour Compression System (VCS): This is more commonly used and comes in three configurations – diffuser type (for low-height rooms), bunker type (cost-effective for tall storage rooms over 11.5 m), and fin coil type (energy-efficient and suitable for rooms from 5.4 m height onwards).

Key parameters in refrigerated storage 🔗

Maintaining the right conditions is critical. Temperature is the single most important factor, but it doesn’t act alone. Relative humidity (RH) must be carefully managed – too high and mould grows; too low and produce dehydrates and wilts. According to Rinac, RH requirements vary significantly: grapes, kiwi, and carrots need 95-98% RH, while onions and garlic require only 65-75%.

Temperature settings also differ by commodity. Tropical and subtropical fruits can suffer chilling injury at temperatures below 12-13°C, resulting in membrane breakdown, enzymatic reactions, and vulnerability to harmful microorganisms. The severity depends on both duration and degree of cold exposure – and damage can occur well above freezing.

Pre-cooling: the first critical step 🔗

Before produce enters cold storage, it often undergoes pre-cooling – a rapid reduction of field heat. The industry standard is called seven-eighth cooling, which means removing 87.5% of the temperature difference between the product and the target storage temperature, typically within 4 to 24 hours. Common pre-cooling methods include room cooling, forced-air cooling, hydro cooling, ice packing, and vacuum cooling. The choice depends on the commodity, packaging, and intended storage duration.

Types of cold storage facilities 🔗

Modern cold storage comes in several forms designed for different needs:

Multi-commodity cold warehouses can store a variety of products at different temperature settings using modular design and racking solutions. Walk-in chillers operate between 2°C and 8°C, while walk-in freezers maintain -18°C to -20°C. For retail and hospitality, display-cum-cold rooms combine product visibility with backend refrigeration. And for transport, reefer trucks maintain the cold chain during distribution.

Controlled atmosphere (CA) storage: going beyond temperature 🔗

While refrigerated storage is effective, some high-value produce needs even more protection to last through long storage periods. This is where controlled atmosphere (CA) storage comes in – a technology that modifies not just the temperature, but the very air surrounding the produce.

The science behind CA storage 🔗

Normal air contains about 78% nitrogen, 21% oxygen, and trace amounts of carbon dioxide (0.04%). In CA storage, the oxygen level is deliberately reduced – often to below 2% – and the CO₂ level is increased, typically to 0-5%. This is done inside gas-tight, sealed chambers where the atmosphere is continuously monitored and adjusted.

By lowering available oxygen, the produce cannot respire as rapidly, which dramatically slows ripening and ageing. As the University of Maryland Extension explains, reducing O₂ from the normal 21% to 1-3% by flushing storage rooms with nitrogen gas significantly slows apple respiration, maintaining firmness, colour, and nutritional content for much longer.

The E+E Elektronik blog notes that CA storage can extend the shelf life of various fruits and vegetables up to four times compared to normal conditions. For example, apples stored under regular cold storage last about 3-4 months, but under CA conditions they remain marketable for 9-12 months.

Key components of CA storage systems 🔗

A functional CA storage facility requires several specialised components working together:

Gas-tight storage rooms: The chambers must be completely sealed to prevent uncontrolled air exchange with the outside environment.

Nitrogen generators: These reduce oxygen levels by introducing nitrogen into the storage room, displacing oxygen from the atmosphere.

CO₂ scrubbers: Since produce releases carbon dioxide during respiration, scrubbers (or bags of hydrated lime) are used to keep CO₂ levels within the target range. Excessive CO₂ can cause browning disorders in the skin and flesh of fruit.

Ethylene management: Ethylene is a gaseous plant hormone that accelerates ripening. In CA storage, ethylene levels must be kept as low as possible. Chemical treatments like 1-methylcyclopropene (1-MCP) can be used to block the fruit’s ethylene receptors, further slowing ripening.

Monitoring sensors: Continuous measurement of O₂, CO₂, temperature, humidity, and ethylene concentration is essential. As Evikon MCI explains, maintaining precise gas levels prevents damage – oxygen below 1% can cause irreversible quality changes, while CO₂ above tolerable limits can harm appearance, taste, and nutritional value.

Which produce benefits most from CA storage? 🔗

CA storage is most commonly and successfully used for apples and pears, where the combination of reduced oxygen and low temperature allows for very long-term storage without major quality loss. However, the technology also works well for kiwifruit, avocados, blueberries, cherries, plums, mangoes, and grapes – each with its own specific gas and temperature requirements.

For instance, pears can be stored for up to 12 months under CA conditions (versus 4 months normally), while cherries extend from about 6 days to 25 days, and avocados from 14 days to 2 months.

Dynamic controlled atmosphere (DCA): the next evolution 🔗

Traditional CA storage maintains fixed gas concentrations throughout the storage period. But produce is not static – its metabolic needs change over time. Dynamic Controlled Atmosphere (DCA) storage addresses this limitation by continuously monitoring the fruit’s physiological response and adjusting oxygen levels in real time.

How DCA works 🔗

DCA technology uses specialised sensors to detect signs of low-oxygen stress in stored fruit. According to USDA-funded research at Cornell University, DCA works by lowering O₂ concentrations to the lowest level tolerated by the fruit without causing injury. This approach reduces or eliminates quality loss associated with ripening during storage.

There are three main types of sensors used in DCA systems:

Chlorophyll fluorescence (CF): The most widely used method, it detects stress in chlorophyll-containing fruit when oxygen drops too low. The system then automatically raises oxygen levels to prevent damage.

Respiration quotient (RQ): Measures the ratio of CO₂ produced to O₂ consumed, providing real-time feedback on metabolic activity.

Ethanol (ET) sensors: Detect ethanol accumulation, which indicates the onset of anaerobic (fermentation) conditions – a signal that oxygen has dropped dangerously low.

Benefits of DCA over traditional CA 🔗

DCA offers several advantages. It maintains better fruit firmness and skin colour compared to standard ultra-low oxygen (ULO) storage. It also reduces the incidence of physiological disorders like superficial scald – a common browning problem in apple storage – potentially eliminating the need for chemical treatments like diphenylamine (DPA).

According to research featured by the EU ENOUGH project, DCA can deliver up to 15% lower energy consumption compared to ULO storage, qualifies as 100% organic (no chemical treatments needed), and helps reduce the fruit sector’s carbon footprint. The estimated CO₂ savings for the Belgian pome fruit sector alone are 1,813-2,720 tonnes annually.

Refrigerated storage vs. CA storage: a comparison 🔗

Both technologies serve the same fundamental goal – extending shelf life – but they operate at different levels of complexity and cost.

Refrigerated storage controls temperature and humidity. It is simpler, less expensive to set up, and suitable for a wide range of produce during short- to medium-term storage. It is the standard for most commercial cold chain operations.

CA storage adds atmospheric control on top of refrigeration. It requires gas-tight rooms, nitrogen generators, CO₂ scrubbers, and continuous monitoring systems. The investment is significantly higher, but the payoff is dramatically extended shelf life – especially for high-value commodities like apples, pears, and kiwifruit that need to be marketed months after harvest.

The choice between the two depends on the type of produce, intended storage duration, market requirements, and available budget. In practice, CA storage always includes refrigeration – it does not replace it, but rather enhances it.

Challenges and future directions 🔗

Despite their benefits, both technologies face challenges. Refrigerated storage requires reliable electricity – a significant barrier in many developing regions. CA storage demands high capital investment and technical expertise for monitoring and maintenance. If gas concentrations deviate even slightly from optimal ranges, produce can suffer irreversible damage.

Emerging trends are addressing these issues. Smart refrigeration systems equipped with IoT sensors now enable real-time remote monitoring of temperature and humidity. Solar-powered cold storage units are being developed for off-grid areas. And DCA technology continues to evolve, with researchers exploring the integration of artificial intelligence and multi-sensor fusion to achieve even more precise atmospheric control tailored to individual fruit batches.

What do you think? As climate change disrupts growing seasons and supply chains, how important will advanced storage technologies become for ensuring year-round food availability? Could affordable, small-scale CA and DCA systems transform postharvest management for smallholder farmers in developing countries?

References

1. https://link.springer.com/article/10.1007/s44187-024-00156-x
2. https://www.postharvest.com/blog/what-is-cold-storage
3. https://rinac.com/blog/a-guide-to-cold-storage-for-vegetables-tips-to-keep-your-produce-fresh/
4. https://extension.umd.edu/resource/controlled-atmosphere-storage-apples
5. https://www.epluse.com/news/blog/detail/2025-09-04-ca-dca-storage-fruit-and-vegetables-stay-fresh-longer-thanks-to-a-controlled-atmosphere/
6. https://www.evikon.eu/news/controlled-atmosphere-in-fruits-and-vegetables-storage-rooms-a-6/
7. https://portal.nifa.usda.gov/web/crisprojectpages/1031509-benefits-of-and-barriers-to-dynamic-controlled-atmosphere-dca-storage-analyses-needed-for-technology-uptake-by-the-us-apple-industry.html
8. https://enough-emissions.eu/non-classe/news-from-demonstrator-5-energy-efficient-dynamic-controlled-atmosphere-dca/